Wireless communication systems become ever more important as they increase user mobility and connectivity. Wireless systems facilitate mobile communication and data exchange in most metropolitan areas. Increasing employment of such systems has resulted in communication traffic handling and power consumption becoming important issues relating thereto.
Code Division Multiple Access (CDMA) allows signals to overlap in both frequency and time. Thus, CDMA signals operate in the same frequency band. More particularly, a scrambling code (e.g., a long pseudo noise code sequence) is associated with each base station and permits the base stations to be distinguished from each other. An orthogonal code (OVSF? code) is allocated to each remote terminal such as for example a mobile station. The OVSF codes are orthogonal with respect to each other, which permits a remote terminal to be distinguished from another. Symbol spreading is accomplished by applying scrambling codes and orthogonal codes at rates higher than the symbol rate (e.g., the chip rate). In IS-95 related systems, pulse-shape filtering is applied to the chips in order to reduce signal interference outside the signal band. However, the pulse shape filters defined in IS-95 systems do not satisfy the Nyquist criterion. Consequently, some interchip interference occurs in IS-95 related systems. This ICI can degrade the bit error rate (BER), particularly in systems employing high order modulation.
Despite various advantages of CDMA, practical issues such as power control speed and inter-base station interference can limit CDMA system(s) effectiveness. A CDMA system depends very much on the ability to provide for accurate power control, but in a mobile environment a communication signal can fluctuate too fast for the system to manage effective control. Additionally, cellular environments are often characterized by unstable signal propagation, severe signal attenuation between communicating entities as well as co-channel interference by other radio sources. Moreover, many urban environments contain a significant number of reflectors (e.g., buildings), causing a signal to follow multiple paths from a transmitter to a receiver. Because separate parts of such a multipath signal can arrive with different phases that destructively interfere, multipath can result in unpredictable signal fading. In addition, in order to provide service to shadowed areas, radiated power is increased, thereby escalating interference between base stations and significantly degrading performance.
Many conventional CDMA systems are multiuser-interference limited, whereas Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) are primarily bandwidth limited. Consequently, in many practical implementations of CDMA, capacity is directly related to signal-to-interference (S/I) ratio, which is essentially a measure of multiuser interference, caused by other overlapping signals.
CDMA cellular and microcellular wireless systems often employ long spreading codes, e.g., sequences with period(s) much longer than data symbol duration, employ complex powerful convolution codes to mitigate effects of multiuser interference and rely on power control strategies to remedy the “near-far” problem. However, as the number of concurrent transmissions increases in a fixed bandwidth system, or as relative power levels of different user signals become disparate (near-far problems), a high performance penalty is observed. The sensitivity of such systems to the multiuser interference and to the near-far problem can substantially mitigate overall system capacity.
In view of the above, it becomes readily apparent that improved cost-effective systems and methodologies are needed for further increasing system capacity and maintaining reasonable S/I ratio so that signal decoding can be carried out efficiently and accurately.